This request is to support an additional and terminal year of study on the nature of acquired immunity to syphilis in rabbits. The plan will be to continue with the studies now in progress with the final objectives of (1) determining the specificity of acquired immunity; (2) identifying the factor(s) in serum that are responsible for its protective power; (3) identifying the various immune forces, both specific and nonspecific, that may contribute to acquired immunity; and (4) evaluating the relative roles of humoral and cellular immunity in acquired immunity.